The present invention relates to a display control method and display controller for displaying data stored in a display memory on a display, and a display apparatus using the same.
As a display apparatus for, e.g., computer equipment, a CRT display device is known. However, since the CRT display device requires a large length in the direction of thickness of the display screen, the entire volume becomes large, and it is difficult to attain a size reduction of the entire system. In a display control of such a CRT display apparatus, display data must always be refreshed using, e.g., a CRT controller (CRTC), resulting in complicated display control.
As displays which can compensate for the drawbacks of the conventional CRT display apparatus, liquid crystal displays, which can attain a size reduction of the display apparatus, in particular, a low-profile structure, are known. Of these liquid crystal displays, a ferroelectric liquid crystal (FLC) has memory characteristics that can maintain an aligned state after an electric field is removed. A display controller for the FLC display (FLCD) need not always refresh the screen unlike in the CRT display controller. In addition, when a display image is changed, the display controller can display the entire changed image by changing only display data of the display corresponding to a portion where the contents of a display memory are changed.
When data is to be displayed on such an FLCD, the refresh period of the display screen can be prolonged unlike a CRT or other displays, thus assuring a sufficient time margin. In addition to refresh control, the display controller for the FLCD requires so-called partial rewrite control for updating display data of only a portion corresponding to a change in image on the display screen. The partial rewrite control is characterized in that it is performed in units of horizontal lines, and is discontinuous in the vertical direction of the display screen.
Upon execution of the partial rewrite control, when the display controller for the FLCD receives a data request signal from the FLCD, it performs a display operation by outputting display data (pixel data) to be displayed and its line address(es) in response to the data request signal. For this reason, when the contents of the display memory have been updated, the display controller must determine line address(es) of the display screen corresponding to the updated portion so as to partially rewrite display data, and must acquire and output the line address to the FLCD.
As a method of determining the line address(es) on a display screen corresponding to the address(es) where the contents of the display memory have been changed, the address may be input to a table comprising, e.g., a RAM, and the corresponding display line address (number) may be output. However, when the address space of the display memory is to be changed in correspondence with the volume of data to be displayed on the FLCD, the change in contents of the display memory cannot be coped with, since a correspondence between the addresses on the display memory and the display lines is not constant. For this reason, when the table is used, the memory space of the display memory must always be fixed.